1. Field of the Invention
This invention relates to an apparatus and method for matching a zero crossing level of a signal reproduced by an information reproducer, and more particularly to a zero crossing level matching apparatus and method that is adapted to process a reproducing signal including two and more components having different zero crossing levels.
2. Description of the Prior Art
Recently, there have been developed and prevalent various types of recording media to record as much information as possible. Examples of such recording media include an optical disc, such as a compact disc(CD) which is relatively large and a digital versatile disc(DVD) which is relatively small, in the width of signal tracks and in the pitch between signal tracks, and so on. In the optical disc, the signal track width and the signal track pitch have been reduced so as to record as much information as possible. Also, in order to reproduce information recorded on the optical disc, a radio frequency signal, hereinafter referred to as "RF signal", read out by means of a pickup must be processed appropriately. In other words, in order to reproduce information on the optical disc, it is necessary to perform a level slicing process for detecting a bit stream (i.e., binaryzation) and a bit stream decoding process including error detection and correction.
Further, the recent recording media include an optical disc in which guiding groove signal tracks are formed to perform tracking easily. In such an optical disc, there exist land signal tracks adjacent to the guiding groove tracks. As shown in FIG. 1, in the optical disc having land and groove signal tracks 2 and 4, embossed pit trains 6 for dividing each of the land and groove signal tracks 2 and 4 into a certain size of unit regions, hereinafter referred to as "sectors", are additionally defined. The embossed pit trains 6 are called "identification code(ID) field" because they indicate the physical position of the signal tracks 2 and 4. On the other hand, the land and groove tracks 2 and 4 divided into sectors by the embossed pit trains 6 are called "recording field" or "user information field" because an information can be recorded thereon by a user. In the disc including the embossed pit trains 6, wobbling signals 8 may be preformatted in each side of the land and groove signal tracks 2 and 4 in such a manner to have the same phase. The wobbling signals 8 are provided by wobbling each side of the land or groove signals track 2 or 4 to have the same phase in accordance with a certain period of carrier signal, as shown in FIG. 2.
As shown in FIG. 3, a conventional information reproducing apparatus for reproducing an information written onto a recording medium, such as CD, DVD and so on, employs an optical pickup 14 to read an information from a recording medium 10 rotated by means of a spindle motor 12. The optical pickup 14 irradiates a light beam onto a land or groove signal track in the recording medium, and converts a light quantity reflected by the signal track into an electrical signal using a photo detector PD. The optical pickup 14 includes a beam splitter BS that allows a light beam from a laser diode LD to be irradiated, via an objective lens OL, onto the land or groove track of the recording medium 10 in a spot shape and allows reflective light received, via the objective lens OL, from the recording medium 10 to be irradiated onto the photo detector PD. The photo detector PD generates electrical signals to be applied to a signal detector 16. The signal detector 16 combines the electrical signals generated at the photo detector PD within the optical pickup 14 to generate a focus error signal Fe, a tracking error signal Te and a radio frequency signal RF. The servo 18 responds to the tracking error signal Te and the focusing error signal Fe from the signal detector 16 and controls a voltage level or a current amount of an actuator driving signal applied to the actuator AC within the optical pickup 14, thereby moving the objective lens OL in the vertical and horizontal direction. Accordingly, a spot-shaped light beam is irradiated onto the land or groove signal track and traces the land or groove signal track. Meanwhile, the channel bit stream detector 20 slices a radio frequency signal RF from the signal detector 16 into a zero crossing level to detect a channel bit stream. To this end, the channel bit stream detector 20 includes a slicer for slicing the radio frequency signal RF into a predetermined voltage level.
When a recording medium shown in FIG. 1 and FIG. 2 is reproduced by means of the conventional information reproducing apparatus, a radio frequency signal RF as shown in FIG. 4, can be generated at the signal detector 16. The radio frequency signal RF includes an embossed pit train or ID component APC having a zero crossing level corresponding to the first voltage level FVL, and a user information component UIC having a zero crossing level corresponding to the second voltage level SVL lower than the first voltage level FVL. This is caused by a fact that the ID field included in the embossed pit train 6 has a light reflectivity higher than the user information field having the land or groove signal track. In other words, when a recording medium of FIG. 1 or FIG. 2 recorded with a user information is reproduced by means of the conventional information reproducing apparatus, a zero crossing level of the radio signal RF generated at the signal detector 16 in the conventional information reproducing apparatus becomes different.
As described above, in order to detect a bit stream for the ID code and a bit stream for the user information from the radio frequency signal having two zero crossing levels, the channel bit stream detector requires two slicing voltage signals having a different voltage level. As a result, the hardware of the channel bit stream detector becomes complex. Also, in the case of performing the digital equalization, the A-D converter has a large response voltage width.